1. Field of the Invention
The present invention relates to Fresnel reflectors and more specifically to spiral Fresnel reflectors.
2. Description of the Prior Art
There is a great need, particularly in the poor third world countries, for an inexpensive solar cooker which can cook food without the necessity of using valuable fuel. Traditionally, solar cookers have been in the form of paraboloids or hemispheres. However, such shapes are difficult to manufacture and in order to keep their shape they must be formed from metal, fiberglass or hard plastic. All of these materials are expensive and the forming processes for these materials are expensive. As a result, the finished cooker is also expensive. Further, such shapes are bulky and require a disproportionate amount of space when shipped. Since shipping volume is expensive, the cost is again increased.
A Fresnel Reflector is a reflector made from a flat sheet of material and having concentric rings which have an identical focal point. The use of a Fresnel reflector as a solar cooker would be advantageous since all of the parts of the Fresnel reflector could be cut from one sheet of material and there would be no complex three dimensional shapes to manufacture; the Fresnel reflector could be made out of inexpensive materials such as aluminized cardboard or aluminized plastic; and the assembled reflector would have a very low profile and would be easy to transport and store. A concentric ring Fresnel reflector is disclosed in "Compact Solar Energy Concentrator" by Robert W. Hosken in Electro-Optical Systems Design, January 1975, pages 32-35. However, the Fresnel reflector described in this article provides rings which are machined into a blank of solid material. Therefore, a high degree of precision is necessary is machining the rings into the blanks with a resulting relatively high cost of manufacture.
A Fresnel reflector using separate concentric rings has been proposed in the past (for example, "EVALUATION OF SOLAR COOKERS" by Volunteers for International Technical Assistance for the U.S. Department of Commerce, Office of Technical Services). However, such a Fresnel reflector using concentric rings has several disadvantages. Each ring of the Fresnel reflector must be assembled and mounted separately, a time consuming task. Further, each reflector is composed of many separate parts. Since each ring is a separate part, there are many parts which can be misplaced or damaged.
An inexpensive, easily transportable, Fresnel reflector would also be advantageous in other areas of solar energy. For example, it could be used for low to medium temperature steam generation for producing power. It could also be used for the production of electric power by use of a Brayton or Sterling cycle generator located at the focus of the reflector. Further, direct electric power production by photovoltaic conversion would also be possible by the placement of solar cells at the focal point of the reflector. Accordingly, an inexpensive, easily transportable, Fresnel reflector would be advantageous in all areas of solar energy production.